High frequency circidits



April (5 w FYL R Re. HIGH FREQUENCY CIRCUITS Original Filed March 24,1956 CARRIER WAVE GENERATER AT MAXIMUM FREQUENCY, REACTANCE VALUE 0! IO-APPROX. Y1, RESISTANCE OF 20, AND RESISTANCE 0F ZO INPUT REACT/"ICE 0FTUBE ll.

FREQUENCY o FREQUENCY I00 v u V0 0 I00 L000 lopoo 0opo0 lpoopooFREQUENCY CYCLES FER. SECOND I50- 7 lab- Ifi m K Dan 40 a 3 660 30 "4 t"a 1 4o- 20k "4 J & m 20 IO a 0 I I I f u h NW3 g g 833%,: g asInventor.

lpoqooo' George F leT, FREQUENCY CYCLES PER. SECOND 29 V H i s Attorneg.

Reissued Apr. 8, 1941 HIGH FREQUENCY CIRCUITS George W. Fylcr,Stratford,

General Electric Company,

York

Conn.. assignor to a corporation of New Original No. 2,190,513, datedFebruary 13, 1940,

Serial No. 70,567,

for reissue January 28, 1941, Serial No.

March 24, 1936. Application Claims. (Cl. 179-171) My invention relatesto high frequency circuits and more particularly to high frequencyapparatus of the short wave high power type. Specifically my inventionpertains to untuned amplifier systems adaptable for use in amplifyingwith a minimum of distortion signal currents having componentfrequencies extending over an exceedingly wide range, such for exampleas signal currents produced by a television signal source wherein thesignal current frequency range extends from a low audio frequency to ahigh radio frequency.

It is an object of my invention to provide an improved amplifier systemcapable of amplifying signal over an exceedingly wide range withsubstantially the same transmission efliciency for all frequencieswithin the range.

It is a further object of my invention to provide an amplifier capableof operating in the above manner in which uniform time delayfor voltagesof all frequencies within. the operating range is produced in eachamplifier stage.

In accordance with my invention I attain the above objects by providingan improved coupling impedance network between successive stages of acascade-connected amplifier which gives a fiat amplifier characteristicand a uniform time delay for all frequencies within. the desiredoperating range. This network includes an inductance connected in serieswith a coupling resistance and having a value sufficient to equalize theamplification of frequencies within the upper portion ofthe signalcurrent frequency range with the amplification of frequencies in the lowportion x of the frequency range. This value has been determined bycalculation and experiments to be sufficient to produce at the highestfrequency which it is desired to amplify an inductive reyactanceapproximately equal to one-half the coupling resistance, where thecouequal to the input impedance value of the pling resistance has avalue reactance into which the coupling network operates.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. .tion itself,however, both as My invento its organization and method of operationtogether with further objects and advantages thereof will best beunderstood by reference to the following specification takeniii-connection with the accompanying drawing in which Fig. 1 illustratesa transmitting system having my invention embodied therein; Fig. 2illustrates currents having frequencies extending the equivalent circuitfor a portion of the circuit shown in Fig. 1; Fig. 3 illustrates agraphical method of determining the correct impedance values for certainof the elements illustrated in Fig. 1; Figs. 4 and 5 illustrate certaincharacteristics of the operation of my invention.

Referring to Fig. 1 of the drawing, I have illustrated my invention asapplied to a television transmitter of conventional design. As shown,the transmitter comprises a radio frequency carrier wave generator Iconnected to supply its output through a driver stage 2,. a push-pullclass C connected power amplifier system 3 and the windings 4 and 5 of acoupling transformer G to the conductors I and 8 of an antennatransmission line system.

The carrier wave output from the power amplifier is modulated inaccordance with the signal oscillations produced by a signal currentsource 9. These signal oscillations are amplifled by the cascade class Aconnected amplifiers Ill, II and I2 and the constant current modulatoramplifier stage l3 and are impressed on the output circuit of the poweramplifier 3 through a modulator coupling impedance network M. Thisnetwork comprises an air core inductor I5 having a very low distributedcapacity shunted by a resistance l6 and connected in series with an ironcore reactor I1 having an inherently high distributed capacity. Oneterminal of the network is tapped to the midpoint iii of the winding 4and the other terminal is connected to the positive side of a highvoltage source (not shown) which is provided to supply energy to therespective anode circuits of the transmitter system.

In order to neutralize the effect of stray capacity in the carrier poweramplifier on the modulator stage an inductance coil I8 is provided whichis connected in the modulator stage plate circuit in the mannerillustrated. It will be understood that such stray capacity is caused bythe distributed capacity of the coils 4 and 5 and also by the effect ofthe neutralizing condensers included in the power amplifier circuit.This stray capacity has been found to be small as compared to thedistributed capacity of the reactor l1 and accordingly the value ofinductance coil l8 may be small.

The source of signal oscillations 9 may oomprise aphoto-electric cellsystem, a source of light, and a scanning disk employed in a televisiontransmitter, all suitably arranged to scan the object to be televisedwith a spot of light. The signal current frequencies which are proandimpressed on the output circuit of the power" amplifier 3 to produce thesame relative degree of modulation. If certain of the frequencies arediscriminated against, as for example, the frequencies in the highportion of the range, frequency distortion results which produces ablurred image or lack of definition when the signals are reproduced in areceiving system.

One problem involved in the construction of a transmitter suitable forthe transmission of a carrier wave modulated in. accordance withtelevision signal currents is that of providing a coupling impedancenetwork l4 between the constant current modulator stage l3 and theoutputcircuit of the amplifier 3 which operates to impress the signalvoltages upon the output circuit without attenuation of the highfrequency components of the impressed voltages. This problem, togetherwith the solution thereof, is completely described in my Patent No.2,147,486, issued February 14, 1939.

Briefly, equalized amplification by the modulation stage I3 of all ofthe components in the side-band frequency range is obtained in accordance with the invention described in the aforementioned patent byproviding the parallel.

connected inductance l5 and resistance It connected in series with theiron core reactor I! across the signal channel and in the anode circuitof the modulator stage H. The constants of this coupling network are soselected that the overall impedance between the terminals thereof ismaintained above a predetermined value for currents of all frequencieswithin the wide frequency range.

Another problem involved in the construction of an amplifying systemcapable of amplifying without substantial distortion of frequencycomponents of the signal voltages is that of obtaining a couplingimpedance network between the successive intermediate amplifier stageswhich is capable of impressing the output voltage of one amplifier stageon the input circuit of the succeeding amplifier stage with equalizedtransmission efflciency for all frequency components contained in thesignal voltages.

In accordance with my invention such equalized transmission efficiencyis obtained by providing in series with each of the coupling resistances2U, 20', etc., inductances l9, l9, etc.,

and so proportioning the impedance constants of these elements withrespect to the input impedance of the amplifier into which the couplingnetwork operates that attenuation of frequencies in the upper portion ofthe substantially eliminated.

The effect of the inductance '9 in the coupling circuit will morereadily be understood by reference to Fig. 2 wherein the equivalentcircuit included between the amplifiers H and I2 is illustrated.

source of electromotive force E; which of course corresponds to theelectromotive force generated sidered as connected in a circuitincluding a re* frequency range is p This equivalent. circuit includes abe determined that the sistance Rp which is the internal staticresistance of the anode circuit of the discharge device II and acondenser C. The circuit also includes an inductance L and resistance R0connected in series across the terminals of the condenser C. Theresistance R0 includes the coupling resistance between the grid andcathode of the discharge device I 2 and comprises the resistance 20 andthe non-inductive resistance of the inductance coil 1 9. The inductanceL of course comprises the inductance of the coil IS. The capacitance Cis the capacitance between the grid and cathode of the discharge deviceI2 and that included in the circuit connections to these electrodes. Theinstantaneous current flowing in the branch circuit including theresistance R0 may be represented by the reference character I1 and theinstantaneous current flowing in the branch circuit including thecapacitance C may be indicated by the reference character I2. Obviouslythe instantaneous current traversing the resistance Rp is equal to thevector sum of I1 and I2.

The value of R0 and of the inductance L necessary to provide a constantratio of Ei/Ez for equalized amplification at all frequencies within thedesired operating frequency range, may be determined by the graphicalmethod illustrated in Fig. 3 from which the curves of Fig. 4 arederived.

In Fig. 3, E2 is drawn to scale along a horizontal axis in the mannerillustrated. This voltage is assumed to be constant in determining therelation between E1 and E2 at any desired frequency. With voltage E2fixed in magnitude and direction, it is necessary to obtain the vectorsum of IlRp and I2Rp at any frequency in order to determine the value ofE1. Since E2 is constant and is the voltage across the series circuit Land R0 at all frequencies, the locus of the vector 11134) as thefrequency is varied from zero to infinity is the semi-circle 2|.Further, the tangent of the angle 6 between the vector IlRp and thehorizontal reference axis is proportional to frequency. By atrigonometric calculation it may distance from 0 to the intersection ofthe projection of the vector 11R, with the vertical reference axis isdirectly proportional to tangent 6. Hence it'will be seen that thelinear scale marked along the vertical axis from 0 upward may directlybe calibrated to read frequency.

Since the circuit through which I: flows is comprised of purecapacitance only, this current is directly proportional to frequency.Hence the linear scale may extend downwardly from 0 and beingproportional to frequency will also be proportional to the vector IZRp,it being understood that this vector is in leading quadrature phaserelation to the voltage vector E2.

With the three vectors E2, IlRp and Iz'R thus determined in phase andmagnitude for any given frequency the voltage E1 at that frequency is ofcourse the vector sum of the three vectors. The angle 0 between thevectors E1 and E2 corresponds to the phase shift between the voltages E:and E1 for the frequency being considered. For

* any given combination of circuit constants the value of El/E2 is ofcourse an are as indicated at sive.

23 having a center at the right end of the vector E: and a radius equalto the sum of the vector E2 and the diameter of the semi-circle 2|.

With the above pictorial representation of the voltage relation betweenthe source voltage E1 and the input voltage E2 fixed for a given set ofcircuit constants the transmission efficiency, or E's/E1, for anyfrequency may be determined, and curves illustrating the relationbetween E2/E1 and frequency may be drawn. Such curves, illustrating therelation between E1, E2 and frequency for different combinations ofcircuit constants, are shown in Fig. 4 by the curves 24 to 29 inclu-Phase shift curves corresponding to the circuit constants which give thetransmission efficiency curves 24 to 29 inclusive, are illustrated at24' to 29' inclusive. In this figure the curves 24 to 29 inclusive showthe transmission efficiencies for the circuit constants given in thefollowing table and the curves 24' to 29' inclusive show thecorresponding phase shift between these voltage vectors as a function offrequency:

Ru L Micro- Gurve ohms henries L 24 and 24' 2000 640 2010 at 500,000cycles. 25 and 25' 2000 240 750 at 500,000 cycles. 26 and 26' 1000 1601000 at 1,000,000 cycles. 27 and 27' 1000 67 420 at 1,000,000 cycles. 28and 28' 00 40 500 at 2,000,000 cycles.

29 and 29 500 10. 7 210 at 2,000,000 cycles C Micro- L Curve microiaradsw c 24 and 24 160 2000 at 500,000 cycles. 25 and 25 160 2000 at 500,000cycles 26 and 20 100 1000 at 1,000,000 cycles 27 and 27'. 100 1000 at1,000,000 cycles. 28 and 28' 100 500 at 2,000,000 cycles. 29 and 20' 100500 at 2,000,000 cycles.

observed that to obtain fiat equalization over the desired frequencyrange it is necessary that the resistance Ru should be approximatelyequal to the capacitive reactance between the input electrodes of thedischarge device into which the coupling impedance network operates atthe highest frequency to be amplified, if equalized amplification overthe entire frequency range is to be obtained. It will further be seenthat the inductance L must be of such value that the reactance thereofat the highest frequency to' be amplified is equal approximately toone-half the value of the resistance R0 which is connected in seriestherewith.

Thus, consider the curves 24 and 25 by way of example, where the desiredfrequency range extends to a value of approximately 500,000 cycles persecond. At this frequency the capacitive reactance of the condenser C isequal approximately to 2000 ohms. inductance value for the inductancecoil IS the reactance thereof is equal approximately to 2010 ohms. Itwill be observed that with this value of inductive reactance a decidedhump is produced in the voltage ratio curve in the upper portion ofUnpre- With 640 microhenries an .the frequency range as shown in curve24. However, if the inductance value of the element l9 be selected to be240 microhenries corresponding to an inductive reactanceof 750 ohms at afre quency of 500,000 cycles per second equalized amplification isobtained, as shown in curve 25, over the entire frequency rangeextending to the desired value of 500,000 cycles per second.

Contrasting curve 25 with curve 21 it will be seen that the latter curveshows that equalized amplification is obtained over a frequency rangeextending to 1,000,000 cycles per second. At this frequency thecapacitive reactance of C is approximately 1000 ohms. From the abovetable of circuit constants the values of resistance Ru and L necessaryto produce this curve are given as 1000 ohms and 67 microhenriesrespectively. Thus the resistance R0 is approximately equal to thecapacitive reactance of C at this upper limit of frequency and theinductive reactance 'of L is 420 ohms or a value approximately equal toone half the value of the coupling resistance R0.

As is noted above the curves 24' to 29 illustrate the phase shiftbetween voltages E1 and E2 as'a' function of frequency. It will ofcourse be understood that phase shift angles may be converted into timedelay intervals representing the time lag between the voltage E1 and E:at selected frequencies within this operating range. For uniform timedelay, the phase shift should be proportional to frequency.

It will further be understood that if phase distortion is to beprevented this time delay should be the same for all frequencies withinthe oper ating frequency range. If the values of phase shift as given bythe curves 25, 21', or 29' for flat or uniform response for anyfrequency be converted into time delay, it will be found that this timedelay is constant for all frequencies within the selected operatingrange covered by the curves. Uniform time delay over a wide band offrequencies in the picture frequency amplifier of a televisiontransmitter does not cause distortion of the transmitted picture signalsfor purposes of reproduction, but simply delays the entire picture. Ihave found experimentally and mathematically calculated that flatequalization of the response characteristic, illustrated for example incurves 25, 21 and 29 of Fig. 4, utilizing the circuits described hereinand therelationship given for the circuit elements and the inputcapacity of the succeeding amplifier, is accompanied by uniform timedelay, illustrated by the phase shift curves 25', 21' and 29 of Fig. 4.In a six-stage amplifier having a flat response over a range extendingfrom 20 to 1,000,000 cycles, the phase shift has been found tocorrespond to a time delay of .53 microsecond.

To further emphasize the importance of selecting the circuit constantsof the coupling network to conform to the principles described above,

' reference may be had to Fig. 5 wherein the amplificationcharacteristics of the signal amplifier system included in thetransmitter shown in Fig. 1 is illustrated. It will be seen thatsubstantially uniform amplification is obtained for all frequenciesextending from 20 cycles to 1,000,000 cycles.

While I have shown a particular embodiment- What I claim as new anddesire to secure by Letters Patent of the United States, is:

1. In combination, an electron discharge amplifier having inputelectrodes comprising a cathode and control grid, a source ofoscillations havthe value of said resistance, whereby substantiallyequal transmission efficiency is obtained for all of said componentfrequencies.

2. In combination, an electron discharge am- .1 plifierhavinginputelectrodes comprising a cathode and a control grid, a source ofoscillations having com-ponent frequencies extending over a wide range,and a coupling impedance network for impressing said oscillations onsaid input electrodes, said network including a resistance having avalue equal to the reactance between said input electrodes at thehighest frequency in said range and an inductance connected in serieswith said resistance, said inductance and resistb ance being in shunt tothe input capacity of said amplifier, said inductance having a reactancevalue at the highest frequency in said range equal approximately toone-half the value of said resistance.

3. In combination, an electron discharge .am-

plifier having input electrodes comprising a cathode and a control grid,a source of oscillations having component frequencies extending over arange from a low audio frequency to a high radio frequency, and animpedance network for impressing said oscillations on said inputelectrodes including a series connected resistanceand inductanceconnected across said source and in shunt to the input capacity of saidamplifier, said resistance having a value equal to the reactance betweensaid input electrodes at the highest frequency in said range and saidinductance having a reactance value at said highest frequency equalapproximately to one-half the value a of said resistance.

4. In a system for amplifying currents having frequencies extending overa wide range and comprising a plurality of cascade-connected amplifiers,a coupling impedance network for impressing said currents from one ofsaid amplifiers on the next succeeding amplifier including a resistancehaving a value approximately equal to the input reactance of thesucceeding stage at the highest frequency to be amplified, and meansfor'equalizing the amplification of currents of all frequencies withinsaid range, said last-named -means including an inductance connected inseries with said resistance, said inductance and resistance beingconnected in shunt to the input capacity of said next succeedingamplifier, said inductance having a reactance value at the highestfrequency to be amplified equal approximately to one-half the resistancevalue of said resist- -ance.

.having component frequencies extending over a wide range, and acoupling impedance network for impressing said oscillations on. saidinput electrodes comprising inductance and resistance connected inseries with each other across said source and in shunt to the inputcapacity of said amplifier, said resistance having a value substantiallyequal to the input reactance of said amplifier at-the highest frequencyin said range and said inductance having a reactance value at thecircuit capacity, and an impedance network connectedin parallel withsaid capacity, saidnetwork including a series connected resistance andinductance, said resistance having a value approximately equal to thereactance of said capacity at the highest frequency in said range-andsaid inductance having a reactance value at said highest frequency equalapproximately to onehalf the value of said resistance.

7. In combination, a source of oscillations having component frequenciesextending over a wide range, a circuit including said source and asubstantially pure capacity reactance, and a branch series circuitincluding said source and a series connected resistance and inductanceconnected across said source and in shunt to said capacity reactance,said inductance and resistance constituting an impedance network forimpressing said oscillations on said capacity reactance, said resistancehaving a value equal to said capacity reactance at the highest frequencyin said range and said inductance having a reactance value at saidhighest value equal approximately to onehalf the value of saidresistance.

8. In combination, a source of oscillations having component frequenciesextending over a wide range, a circuit including a total input capacityreactance, and a coupling impedance network for impressing saidoscillations on said capacity reactance including a resistance having avalue approximately equal to said capacity reactance at the highestfrequency to be transmitted and an inductance connected in series withsaid resistance, said inductance and resistance being connected in shuntto said capacity reactance, said inductance having a reactance value atthe highest frequency to be transmitted equal approximately to one-halfthe value of said resistance.

9.-In combination, a source of oscillations including an electrondischarge amplifier, said oscillations having component frequenciesextending over a wide range, a circuit including a total input capacity,a coupling impedance network for impressing said oscillations from saidamplifier on said capacity including a resistance having a valueapproximately equal to the reactance of said capacity at the highestfrequency to be amplified, and means for equaiizing the amplification ofcurrents of all frequencies within said range, saidlast-named meansincluding an inductance connected in series with said resistance, saidinductance and resistance being connected in shunt to said capacity,said inductance having a reactance value at the highest frequency to beamplified equal approximately to one-half the value of said resistance.

10. In combination, a source of oscillations extending over a Widefrequency band, said source having an internal regulation impedance, apair of, output terminals, means for connecting said oscillation sourceto said terminals, a succeeding utilization circuit, means forconnecting said utilization circuit to said terminals, the group ofelements comprising said oscillation source, said connection means, andsaid utilization circuit including a total capacity in shunt with saidterminals, and a coupling impedance network connected in shunt with saidterminals, said net- 10 work being constituted by a resistance having avalue approximately equal to the total effective reactance of saidcapacity at the highest frequency in said band and an inductanceconnected in series with said resistance, the reactance of saidinductance at said highest frequency having a value equal approximatelyto one-half the value of said resistance.

GEORGE W. FYLER.

